Capacitor-less memory cell, device, system and method of making same

ABSTRACT

A capacitor-less memory cell, memory device, system and process of forming the capacitor-less memory cell includes forming the memory cell in an active area of a substantially physically isolated portion of the bulk semiconductor substrate. A pass transistor is formed on the active area for coupling with a word line. The capacitor-less memory cell further includes a read/write enable transistor vertically configured along at least one vertical side of the active area and operable during a reading of a logic state with the logic state being stored as charge in a floating body area of the active area, causing different determinable threshold voltages for the pass transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/073,624, filed Mar. 28, 2011, now U.S. Patent 8,203,866, issued Jun.19, 2012, which is a divisional of U.S. patent application Ser. No.11/711,449, filed Feb. 26, 2007, now U.S. Pat. No. 7,919,800, issuedApr. 5, 2011, the disclosure of each of which is hereby incorporatedherein by this reference in its entirety.

FIELD OF THE INVENTION

Various embodiments of the present invention relate generally to thefield of volatile memory devices and, more particularly, tocapacitor-less memory cells.

BACKGROUND OF THE INVENTION

A widely utilized DRAM (Dynamic Random Access Memory) manufacturingprocess utilizes CMOS (Complementary Metal Oxide Semiconductor)technology to produce DRAM circuits, which comprise an array of unitmemory cells, each including one capacitor and one transistor, such as afield effect transistor. In the most common circuit designs, one side ofthe transistor is connected to one side of the capacitor, the other sideof the transistor and the transistor gate are connected to externalcircuit lines called the digit line and the word line, and the otherside of the capacitor is connected to a reference voltage. In suchmemory cells, an electrical signal charge is stored in a storage node ofthe capacitor connected to the transistor that charges and dischargesthe circuit lines of the capacitor.

Higher performance, lower cost, increased miniaturization of components,and greater packaging density of integrated circuits are ongoing goalsof the computer industry. In pursuit of increased miniaturization, DRAMchips have been continually redesigned to achieve ever higher degrees ofintegration. However, as the dimensions of the DRAM chips are reduced,the occupation area of each unit memory cell of the DRAM chips must bereduced. This reduction in occupied area necessarily results in areduction of the dimensions of the capacitor, which, in turn, makes itdifficult to ensure required storage capacitance for transmitting adesired signal without malfunction. However, the ability to densely packthe unit memory cells, while maintaining required capacitance levels,results in the necessity to build taller or deeper capacitors in orderto maintain adequate charge storage for adequate data retention.Accordingly, taller or deeper capacitors results in aspect ratios thatrequire expensive processes and result in increased opportunities fordefects.

Specialized fabrication processes unique to the formation of largeaspect ratio devices such as capacitors do not lend themselves to beingintegrated with logic devices such as controllers or processors.Therefore, it would be advantageous to develop a data storage cellcapable of high-density fabrication while not utilizing overly peculiarprocessing steps that are incompatible with logic device fabricationtechniques.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross section of a formation of a structure, in accordancewith an embodiment of the present invention.

FIG. 2 is a cross section of a further formation of the structure ofFIG. 1, in accordance with an embodiment of the present invention.

FIG. 3 is a cross section of a further formation of the structure ofFIG. 2, in accordance with an embodiment of the present invention.

FIG. 4 is a cross section of a further formation of the structure ofFIG. 3, in accordance with an embodiment of the present invention.

FIG. 5 is a cross section of a further formation of the structure ofFIG. 4, in accordance with an embodiment of the present invention.

FIG. 6 is a cross section of a further formation of the structure ofFIG. 5, in accordance with an embodiment of the present invention.

FIG. 7 is a cross section of a further formation of the structure ofFIG. 6, in accordance with an embodiment of the present invention.

FIG. 8 is a cross section of a further formation of the structure ofFIG. 7, in accordance with an embodiment of the present invention.

FIG. 9 is a cross section of a further formation of the structure ofFIG. 8, in accordance with an embodiment of the present invention.

FIG. 10 is a cross section of a further formation of the structure ofFIG. 9, in accordance with an embodiment of the present invention.

FIG. 11 is a cross section of a further formation of the structure ofFIG. 10, in accordance with an embodiment of the present invention.

FIG. 12 is a circuit diagram of a capacitor-less memory cell, inaccordance with an embodiment of the present invention.

FIG. 13 is a block diagram of a memory device, in accordance with anembodiment of the present invention.

FIG. 14 is a block diagram of an electronic system, in accordance withan embodiment of the present invention.

FIG. 15 is a block diagram of an electronic system, in accordance withanother embodiment of the present invention.

FIG. 16 is a diagram of a semiconductor wafer including an integratedcircuit die incorporating a memory cell of one or more of the previousembodiments, in accordance with a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be implemented, and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

Embodiments of a capacitor-less memory cell, memory device, system andprocess of forming the capacitor-less memory cell are disclosed. Thecapacitor-less memory cell is formed according to a partialsilicon-on-insulator (SOI) technique, wherein the active area is formedfrom a substantially physically isolated portion of the bulksemiconductor substrate. A pass transistor is formed on the active areaand includes a pass transistor including a source region and a drainregion for coupling with a digit line. A gate of the pass transistor isconfigured for coupling with a word line. The capacitor-less memory cellfurther includes a read/write enable transistor including a gate, asource region and a drain region commonly shared with the source regionof the pass transistor. The read/write enable transistor is verticallyconfigured along at least one vertical side of the active area andactivated, or operable, during a reading of a logic state with the logicstate being stored as charge in a floating body area of the active area,causing different determinable threshold voltages for the passtransistor.

An embodiment of a process for forming a capacitor-less memory cell isalso disclosed. The process includes etching an active area from a bulksemiconductor substrate with the active area being substantiallyphysically isolated from the bulk semiconductor substrate in a partialSOI process. A read/write enable transistor is formed and includes agate, a source region and a drain region commonly shared with the sourceregion of the pass transistor. The read/write enable transistor isvertically configured along at least one vertical side of the activearea and configured to be activated, or operable, during a reading of alogic state. The process further includes forming a pass transistor inthe active area with the pass transistor including a source region, adrain region for coupling with a digit line and a gate for coupling witha word line. A logic state is stored as charge in a floating body areaof the active area, causing different determinable threshold voltagesfor the pass transistor.

An embodiment of a memory device is also disclosed, which embodimentincludes a memory array including a plurality of capacitor-less memorycells. Each of the plurality of capacitor-less memory cells includes anactive area formed from a substantially physically isolated portion of abulk semiconductor substrate with a pass transistor formed on the activearea. The pass transistor includes a source region, a drain region forcoupling with a digit line and a gate for coupling with a word line.Each of the plurality of capacitor-less memory cells further includes aread/write enable transistor including a gate, a source region and adrain region commonly shared with the source region of the passtransistor. The read/write enable transistor is vertically configuredalong at least one vertical side of the active area and activated, oroperable, during a reading of a logic state with the logic state beingstored as charge in a floating body area of the active area, causingdifferent determinable threshold voltages for the pass transistor. Thememory device also includes addressing and sensing circuitry coupled tothe memory array and configured to select and read and write to selectedones of the plurality of capacitor-less memory cells.

An embodiment of a semiconductor wafer including at least one memorydevice including a plurality of capacitor-less memory cells thereon isalso disclosed.

Embodiments of electronic systems including input, output, processor andmemory devices are also disclosed. In one embodiment of the presentinvention, the electronic system includes the input, output, processorand memory devices operably coupled together. In another embodiment ofthe present invention, the input, output, and processor devices areoperably coupled together and the memory device is integrated into theprocessor device. The capacitor-less memory cells are further formed andconfigured as described herein.

In one embodiment as depicted in FIG. 1, a substrate 10 is provided,which includes a semiconductive material. The terms “wafer” and“substrate” used in the following description include any structurehaving an exposed surface, on or in which an integrated circuit (IC)structure relating to embodiments of the present invention may beformed. The term substrate includes, without limitation, semiconductorwafers. The term substrate is also used to refer to semiconductorstructures during processing, and may include other layers that havebeen fabricated thereupon. Both wafer and substrate include doped andundoped semiconductors, epitaxial semiconductor layers supported by abase semiconductor or insulator, as well as other semiconductorstructures known to one skilled in the art. The term “conductor”includes semiconductors, and the term “insulator” or “dielectric”includes any material that is less electrically conductive than thematerials referred to as conductors.

The illustrated portion of the substrate 10 may also be a portion of animplanted “tub” region of, for example, a p-type doped region of adifferently doped greater substrate. The substrate 10 has an implantregion 12 formed through ion implantation into the substrate 10according to conventional implant techniques, including masking, to forman implant region that is offset in one direction as illustrated and, inone embodiment, implant region 12 is formed to result in an n-typeregion. The substrate 10 also includes a pad oxide layer 14 depositedthereon. As used herein, the term “deposited” is used broadly to meanlayers that are not only deposited in the traditional sense, but layersof material that are grown or in any other manner caused to be formed. Aprotective layer 16 is deposited on top of the pad oxide layer 14 to actas a buffer during subsequent etch steps and other processing. In oneembodiment, the protective layer 16 is polysilicon. In anotherembodiment, the protective layer 16 is a nitride material. In yetanother embodiment, the protective layer 16 is a polysilicon layer thatis covered with a nitride material. The specific combination is selecteddepending upon process integration choices.

A mask 18 is formed and patterned upon the protective layer 16. In oneembodiment, the mask 18 is a photoresist material that is spun on,exposed, cured, and patterned. In another embodiment, the mask 18 is ahard mask material such as a nitride or oxide. The area protected by themask 18 defines what will become an active area in a partialsilicon-on-insulator (SOI) structure used to form a capacitor-lessmemory cell.

FIG. 2 illustrates an embodiment after an etch process that has exposedthe regions unprotected by the mask 18. In the etch process, theprotective layer 16 and the pad oxide layer 14 have also been patterned,and a recess 20 has been formed with a recess first bottom 22 and anupper first wall 24 and an upper second wall 26. It should be noted thatonly a cross section of the structure is illustrated in FIGS. 1 through11 and therefore upper third and fourth walls are not illustrated butare located on adjacent sides of upper first wall 24 and upper secondwall 26.

FIG. 3 illustrates the structure depicted in FIG. 2 after furtherprocessing in which the mask 18 has been removed and a nitride film 28has been grown onto the exposed semiconductive material of the substrate10. In one embodiment, the exposed semiconductive material of thesubstrate 10 is exposed silicon. The nitride film 28 is depicted ascovering the recess first bottom 22, the upper first wall 24 and uppersecond wall 26. The nitride film 28 may be grown by known processesunder conditions that deposit only upon semiconductive material such asexposed silicon. One such process is remote-plasma nitridation (RPN). InRPN, a nitride-bearing plasma is struck, remote from substrate 10, butwithin the deposition tool, and the nitride-bearing plasma is carried byconvective force toward the substrate 10. Another process that may beused to faun the nitride film 28 is rapid thermal nitridation (RTN).Such processing is also known in the art.

Alternative to the formation of a nitride film 28, an oxide film may beformed, either by remote-plasma oxidation (RPO) or by rapid thermaloxidation (RTO) or in situ steam generation (ISSG) or low-pressureradical oxidation (LPRO). Similarly, a combination of an oxide and anitride is formed according to an embodiment as set forth herein. In oneembodiment, the placement of the oxide precedes the placement of thenitride, or vice versa. Similarly, an oxynitride film is formed in theplace of the nitride film 28 according to an alternative embodiment. Theprocess is carried out by either a remote plasma process or a rapidthermal process. Although not limiting the embodiments disclosed, forconvenience throughout the remainder of the disclosure, the film 28 isreferred to as the nitride film 28.

FIG. 4 illustrates processing of the substrate 10 in which an etch hasformed a recess second bottom 30 below the level of the recess firstbottom 22 and at about the depth of the implant region 12. The recessfirst bottom 22 now appears as a substrate ledge structure. Because ofthe presence of the nitride film 28, the upper first wall 24, uppersecond wall 26, and upper third and fourth walls (not shown) areprotected, and a lower wall 32 has been formed that is approximatelycoplanar with the lateral extremity of the nitride film 28. In oneembodiment, an anisotropic etch, such as a reactive ion etch, is usedsuch that the nitride film 28 is left standing upon the ledge of what isleft of the recess first bottom 22.

For a 0.25 micron critical-dimension (CD or minimum feature) process,the remnant of the nitride film 28 has a height in a range from about0.1 micron to about 0.15 micron. In this dimension, the distance fromthe remnant of the recess first bottom 22 to the recess second bottom 30is in a range from about 0.1 micron to about 0.3 micron. Alternatively,for a 0.15 micron critical-dimension (CD or minimum feature) process,the remnant of the nitride film 28 has a height, H, in a range fromabout 0.07 micron to about 0.12 micron. In this dimension, the distancefrom the remnant of the recess first bottom 22 to the recess secondbottom 30 is in a range from about 0.08 micron to about 0.2 micron.

At the level of the recess second bottom 30, a deep implantation region34 is formed. In one embodiment, the deep implantation region 34 is madeof materials that are substantially identical to the bulk semiconductivematerial in the substrate 10. Implantation is carried out at an energylevel that achieves self-interstitial implantation, and that causes theimplantation region 34 to become amorphous enough to have an etchresponsiveness that is different from the bulk semiconductive materialin the substrate 10. In one embodiment, implantation conditions use asilicon source that is implanted to a monocrystalline-to-selfinterstitial ratio of about 3:1. By “silicon source” it is meant thatsilicon or another Group IV element is used, or a combination such assilicon and germanium. In one embodiment, the implanted concentration isfrom about 1 E¹⁴ atoms/cm² to about 5 E¹⁵ atoms/cm² at processconditions of ambient temperature (20° C. to about 30° C.) and animplantation energy from about 500 eV to about 30 KeV. In oneembodiment, a silicon source that is substantially equivalent to thesilicon chemistry of the bulk of the semiconductive substrate 10, isimplanted to a concentration of about 1 E¹⁵ atoms/cm² and processconditions are about 25° C. and an implantation energy of about 25 KeV.In another embodiment, the implantation energy may be on the order ofabout 1 KeV.

After the deep implantation, an etch process is used in subsequentprocessing that is selective to the amorphous material of theimplantation region 34 and to the nitride film 28, but the etch processremoves bulk semiconductive material in the substrate 10. In oneembodiment, the etch process is a wet tetramethyl ammonium hydroxide(TMAH) etch as is known in the art. In another embodiment, the wet etchuses a potassium hydroxide (KOH) etch chemistry that is known in theart. The TMAH etch chemistry is desirable because it is selective suchthat it etches the bulk silicon of the substrate 10, but does notsubstantially etch the nitride film 28 or the deep implantation region34. In one embodiment, the selectivity is in a range from about 5:1 toabout 20:1. In another embodiment, the selectivity is about 10:1. Theisotropic etch may also be combined with an anisotropic etch, eitherbefore or after the isotropic etch. By using both an isotropic and ananisotropic etch, both the downward etching and the undercutting of thenitride film 28 may be varied to suit particular applications.

Various wet TMAH etch processes are known that are selective toamorphous silicon and to nitride films (or oxide films, or oxynitridefilms), and that isotropically etch bulk monocrystalline silicon alongcrystallographic planes. FIG. 5 illustrates the results of a TMAH etchthat has formed a lateral cavity 38 that has undercut what will becomethe active area 36. By this undercutting etch, the active area 36 hasbeen mostly isolated from the bulk semiconductive material in thesubstrate 10, at the level of the ledge that is formed at the recessfirst bottom 22.

Under the etch conditions, and due to the scale of the lateral cavity38, a distinctive contour is formed therein. The TMAH etch has an effectalong crystallographic planes such that a faceted contour may appearwithin the lateral cavity 38. Accordingly, faceted surface 44 isillustrated on one side. However, these are depicted in arbitrary shape,angle and size for illustrative purposes, and the specific shapes,angles, and sizes of the faceted surfaces will depend upon thecrystallographic orientation of the bulk semiconductive material in thesubstrate 10 and upon the specific etch process and conditions, amongother factors. According to the specific etch conditions, aphotomicrographic view of the lateral cavity 38 depicts subtendedcrystallographic planes of bulk semiconductive material in the substrate10 that have been exposed by the TMAH etch. It should be noted thatthere are other various methods for forming the lateral cavity 38, whichare also contemplated to be within the scope of the present invention.

After formation of the lateral cavity 38, the deep implantation region34 is treated to form an annealed implantation region 46 as illustratedin FIG. 6. The annealed implantation region 46 has been returned tosubstantially the same semiconductive quality as the bulk semiconductivematerial in the substrate 10 by substantially repairing themonocrystalline lattice in what was the deep implantation region 34(FIG. 5). The conditions for annealing are known in the art and dependupon the depth of the deep implantation region 34, the available thermalbudget of the process, and other factors.

FIG. 7 illustrates further processing according to an embodiment. In oneembodiment, the exposed surface of the active area 36 and the bulksemiconductive material of the substrate 10 is oxidized, in oneembodiment, using minimal conditions. The minimal oxidation conditionsrelate to a lowered workpiece stress in the lateral cavity 38. Anoxidation 48, such as a shallow trench isolation (STI) oxide, is formedthat provides a thin oxide layer. The oxidation 48 consumes silicondownward into the substrate 10, sideways into the faceted area 44 (FIG.6), and upward into bottom of the active area 36. In onephotolithographic process, such as a 0.25 micron process, the dimensionsare about 0.03 micron growth of oxidation 48 toward the remainingportion of a substrate stem 52. In another photolithographic process,such as a 0.15 micron process, the dimensions are about 0.01 microntoward the substrate stem 52 that remains to this stage of processing.

FIG. 7 also depicts the protective layer 16 remaining while the nitridefilm 28 has been removed. This embodiment occurs where the protectivelayer 16 is chemically different from the nitride film 28, such as apolysilicon protective layer 16. In another embodiment, where theprotective layer 16 is a nitride material, it is removed with thenitride film.

For one photolithographic process, the amount of the substrate 10 thatis consumed sideways by the isotropic etch, for example, isapproximately 0.07 micron on each side of the active area 36. Thatoxidation process leaves the substrate stem 52, which connects thesubstrate that will become the active area 36 to the bulk of thesubstrate 10. In this embodiment, the substrate stem 52 is on the orderof about 0.05 micron by 0.05 micron. Oxidation time will depend upon thearea of the partially isolated structure that forms the active area 36and the other parameters. In one embodiment, oxidation parametersinclude a processing temperature from about 850° C. to about 1,100° C.The ambient is with wet or dry oxygen (O₂) or radicals or ozone,atmospheric pressure or higher. In one example, a temperature of about850° C. and a wet oxygen ambient is applied for a sufficient time toallow about 0.03 micron horizontal oxidation under the active area 36,and about 0.01 micron vertical oxidation upwardly into the active area36. After the thermal oxidation process, oxidation is formed to filllateral cavity 38 and provide support and isolation to the active area36 supported by substrate stem 52.

In one embodiment, a first oxide 40 is formed for filling the lateralcavity 38. The first oxide 40 may be formed of a spin-on dielectric(SOD) material, high-density plasma (HDP) oxide material or otherdielectric fill. When a SOD material is desired, the layer 48 may beconfigured as a nitride and oxide combination layer to allow the properdensification of the SOD material. By way of example, spin-on dielectricoxide (SOD) material provides good oxidation for trenches or cavities,such as lateral cavity 38, which are formed according to sub-microndimensions. The spin-on dielectric (SOD) process entails dripping aliquid precursor onto the wafer surface in a predetermined amount. Thewafer is subjected to rapid spinning (e.g., up to 6000 rpm). Thespinning uniformly distributes the liquid on the surface by centrifugalforces allowing low points to be filled. Finally, the coating is bakedin order to solidify the material. Further details of spin-on dielectric(SOD) processes are known by those of ordinary skill in the art and mayinclude processes described in U.S. Pat. No. 7,112,513, the disclosureof which is incorporated herein by reference. In yet another embodiment,a TEOS material may be used instead of a SOD material. Furthermore,combinations of the oxide materials are also contemplated.

While a continuous fill of oxide material into the cavity 38 for forminga planar isolation to the protective layer 16 is contemplated, thepresent embodiment illustrates a second oxide 42 formed by an oxidationprocess (e.g., high-density plasma (HDP) oxide material, spin-ondielectric (SOD) material or other dielectric fill). The second oxide 42may contain the same dielectric material or a dielectric materialdifferent from the first oxide 40. Since the lateral cavity 38 has beenfilled by the first oxide 40, the second oxide 42 may be formedaccording to more aggressive oxidation processes.

FIG. 8 illustrates further processing according to an embodiment of thepresent invention. A mask 50 is formed and patterned upon the protectivelayer 16 and the second oxide 42 on one side of the active area 36. Inone embodiment, the mask 50 is a photoresist material that is spun-on,exposed, cured, and patterned. The mask 50 protects one or more sides ofthe active area 36 from process steps occurring on at least one otherside of active area 36. The mask 50 provides protection from removal ofthe first oxide 40 and second oxide 42 on at least one side of theactive area 36. An etching process exposes the regions unprotected bythe mask 50. In the present etch process, the protective layer 16 andthe mask 50 allow the first oxide 40′ and the second oxide 42′ to beremoved for the formation of a vertical gate along at least one side ofthe active area 36.

FIG. 9 illustrates further processing according to an embodiment of thepresent invention. A minimal oxidation in the form of gate oxide 54 isformed along an open upper second wall 26 of active area 36 and along anopen cavity wall 56 of substrate stem 52. The continuous gate oxide 54allows for the formation of a vertical transistor along active area 36and substrate stem 52. A conductive material 60 is formed over the gateoxide 54 to create the vertical gate 58 on at least one side of thesubstrate stem 52. Furthermore, an n-type junction extension 61 isdiffused from the poly fill area, which results in providing a desirableelectrical contact between conductive material 60 and the implant region12. In one embodiment, conductive material 60 (e.g., polysilicon ormetal) forms a continuous conductor along the z-direction (i.e., intoand out of the illustrated figure and parallel with the word line, whichis perpendicular with the digit line) for coupling with a read/writeenable signal 62. Furthermore, the proximity of adjacent memory cellsmay be reduced by placing a single contact at the end of the continuousconductor of the read/write enable signal 62.

FIG. 10 illustrates further processing of the structure, according to anembodiment of the present invention. The active area 36 is implantedaccording to mask 64 to form a first drain region 66 and first sourceregion 68, configured as a floating source region, of a forthcoming passtransistor 70. The pass transistor 70 is gated by a word line and thefirst drain region 66 is connected to a digit line for reading andwriting by a sense amplifier (not shown). The active area 36 is furtherimplanted according to mask 72 to form a second drain region 74, whichin combination with first source region 68, forms the drain region,referred to hereafter as a common region 80, for the verticallyconfigured read/write enable transistor 76. A second source region 78for the vertically configured read/write enable transistor 76 resultsfrom the implant region 12.

FIG. 11 illustrates further processing of the structure, according to anembodiment of the present invention. A capacitor-less memory cell 82including a pass transistor 70 and a read/write enable verticaltransistor 76 is formed on the active area 36 according to a partial SOIprocess. The pass transistor 70 at the first drain region 66 couples toa digit line 84 and at the gate 86 couples to the word line 88. Thesource region of the pass transistor 70 is configured as a floatingsource at common region 80. The vertically configured read/write enabletransistor 76 includes the vertical gate 58, a drain region located atthe common region 80 and the second source region 78 resulting fromimplant region 12 (FIG. 1). The vertical gate 58 couples to theread/write enable signal 62, which is activated during reading of thecapacitor-less memory cell 82.

FIG. 12 illustrates a circuit diagram of the capacitor-less memory cell82, in accordance with an embodiment of the present invention. Thecapacitor-less memory cell 82 is illustrated to include pass transistor70 coupled with the read/write enable transistor 76. The various controlsignals, namely word line 88 and read/write enable signal 62,respectively, control the gates of the pass transistor 70 and theread/write enable transistor 76. The digit line 84 is coupled to thefirst drain region 66 of pass transistor 70. A bipolar junctiontransistor (BJT) 90 is also illustrated as the dominant parasiticdevice. In general, information is stored in the capacitor-less memorycell 82 by charging or discharging the channel region of the passtransistor 70. The channel region of the pass transistor 70 is isolatedaccording to the partial SOI process described hereinabove and inaddition to the gated-diode configuration of the read/write enabletransistor 76. Such a configuration results in a low leakage from thechannel region and accommodates charge storage in the channel region.

During operation when the capacitor-less memory cell is neither beingwritten to or read from, the read/write enable signal 62 is set at lessthan the threshold voltage of the vertical read/write enable transistor76 but greater than the VBE of about, for example, 0.55 V. First andsecond logic states are written into the capacitor-less memory cellaccording to charge stored in the floating body region 92 (FIGS. 11 and12).

A first logic state, such as a low logic state, is written into thecapacitor-less memory cell by storing charge in the floating body region92. The charging of the floating body region 92 occurs when theread/write enable transistor 76 is turned off by applying a negativevoltage (e.g., about −0.6V to −1V) as the read/write enable signal 62.The p-type substrate 10 is set to 0V or allowed to float. This conditionresults in low conduction through the pass transistor 70 during a readoperation since the threshold voltage of the pass transistor is higher(e.g., about +1V) and further since the VBB<<VBE as charge has beenadded to the floating body region 92.

A second logic state, such as a high-logic state, is written into thecapacitor-less memory cell by depleting or discharging the charge fromthe floating body region 92. Discharging occurs when the read/writeenable transistor 76 is turned off by applying a positive voltage (e.g.,about +0.6V) to the read/write enable signal 62 and applying a positivevoltage (e.g., about +1V) to the p-type substrate 10. This conditionresults in a high conduction through the pass transistor 70 during aread operation since the threshold voltage of the pass transistor islower (e.g., about 0.2V) and further since the VBB≈VBE−0.1V as chargehas been discharged from the floating body region 92.

During a read operation of the capacitor-less memory cell, the logicstate is read to the digit line 84 when the word line 88 is high (i.e.,between the threshold voltage of the pass transistor indicating ahigh-logic state and the threshold voltage of the pass transistorindicting a low-logic state). Additionally, the read/write enabletransistor 76 is turned on by applying a voltage to the read/writeenable signal 62 that is greater than the threshold voltage of theread/write enable transistor 76 and the p-type substrate 10 is floating.

Since there is finite leakage in the floating body region 92, adequateelectron concentrations must be maintained in order to preserve thestored logic states. Therefore, the capacitor-less memory cell may becharacterized as a form of a Dynamic Random Access Memory (DRAM).Accordingly, refresh operations need to be periodically performed withthe period being determined based upon the specific processesimplemented.

FIG. 13 is a block diagram of a memory device, in accordance with anembodiment of the present invention. A DRAM memory device 100 includescontrol logic circuit 120 to control read, write, erase and performother memory operations. A column address buffer 124 and a row addressbuffer 128 are adapted to receive memory address requests. A refreshcontroller/counter 126 is coupled to the row address buffer 128 tocontrol the refresh of the memory array 122. A row decode circuit 130 iscoupled between the row address buffer 128 and the memory array 122. Acolumn decode circuit 132 is coupled to the column address buffer 124.Sense amplifiers-I/O gating circuit 134 is coupled between the columndecode circuit 132 and the memory array 122. The DRAM memory device 100is also illustrated as having an output buffer 136 and an input buffer138. An external processor may be coupled to the control logic circuit120 of the DRAM memory device 100 to provide external commands.

A capacitor-less memory cell 150 of the memory array 122 is shown inFIG. 13 to illustrate how associated memory cells are implemented in thepresent invention. States or charges are stored in the capacitor-lessmemory cell 150 that correspond to a data bit. A word line WL0 142 iscoupled to the gate of the pass transistor of the capacitor-less memorycell 150. When the word line WL0 142 is activated, the charge stored inthe capacitor-less memory cell 150 causes a determinable amount ofcurrent to flow or to not flow to the digit line DL0 152 based upon thelogic state stored in the capacitor-less memory cell. Digit line DL0 152is coupled to a sense amplifier in circuit 134.

FIG. 14 is a block diagram of an electronic system, in accordance withan embodiment of the present invention. The electronic system 200includes an input device 272, an output device 274, and a memory device278, all coupled to a processor device 276. The memory device 278incorporates at least one capacitor-less memory cell 240 of one or moreof the preceding embodiments of the present invention.

FIG. 15 is a block diagram of an electronic system, in accordance withanother embodiment of the present invention. The electronic system 300includes an input device 272, an output device 274, and a processordevice 376 incorporating therein a memory device 378, which includes atleast one capacitor-less memory cell 340. As stated, the disclosedcapacitor-less memory cell implemented using a partial SOI process doesnot require the process fabrication steps for forming large dataretention capacitors. Therefore, the fabrication processes for formingthe capacitor-less memory device, according to the embodiments describedherein, are compatible with the fabrication processes for forming logicdevices such as a processor device. Accordingly, a memory deviceincorporating the capacitor-less memory cell described herein may beintegrated onto the processor die for close integration.

FIG. 16 is a diagram of a semiconductor wafer including an integratedcircuit die incorporating the memory array and capacitor-less memorycells of one or more of the previous embodiments, in accordance with afurther embodiment of the present invention. As shown in FIG. 16, asemiconductor wafer 400 includes a yet-to-be cut integrated circuit die440 that incorporates one or more capacitor-less memory cells as hereindisclosed.

The processes and devices described above illustrate embodiments ofmethods and devices out of many that may be used and produced accordingto the embodiments of the present invention. The above description anddrawings illustrate embodiments that provide significant features andadvantages of the present invention. It is not intended, however, thatthe present invention be strictly limited to the above-described andillustrated embodiments.

Although the present invention has been shown and described withreference to particular embodiments, various additions, deletions andmodifications that will be apparent to a person of ordinary skill in theart to which the invention pertains, even if not shown or specificallydescribed herein, are deemed to lie within the scope of the invention asencompassed by the following claims.

What is claimed is:
 1. A process of forming a capacitor-less memorycell, comprising: forming an active area from a bulk semiconductorsubstrate to be substantially physically isolated from the bulksemiconductor substrate by at least one material extending laterallybetween the active area and the bulk semiconductor substrate; forming apass transistor in the active area, the pass transistor including asource region, and a drain region for coupling with a digit line and agate for coupling with a word line; and forming a read/write enabletransistor vertically configured along at least one vertical side of theactive area and including a gate, a source region, and a drain regioncommonly shared with the source region of the pass transistor.
 2. Theprocess of forming a capacitor-less memory cell of claim 1, wherein theforming the active area further comprises removing a portion of the bulksemiconductor substrate to form a lateral cavity below the active area,and retaining a stem portion of the bulk semiconductor substrate tophysically integrally connect the active area with the bulksemiconductor substrate.
 3. The process of forming a capacitor-lessmemory cell of claim 2, wherein forming the pass transistor furthercomprises forming at least a portion of the gate of the read/writeenable transistor along at least one side of the stem portion.
 4. Theprocess of forming a capacitor-less memory cell of claim 3, furthercomprising forming oxide isolation on at least another side of the stemportion between the active area and the bulk semiconductor substrate. 5.The process of forming a capacitor-less memory cell of claim 2, furthercomprising implanting the source region of the read/write enabletransistor prior to removing the portion of the bulk semiconductorsubstrate to form the lateral cavity.
 6. The process of forming acapacitor-less memory cell of claim 5, further comprising locating thesource region of the read/write enable transistor in the bulksemiconductor substrate under the stem portion.
 7. The process offorming a capacitor-less memory cell of claim 1, further comprisingforming a commonly shared source region of the pass transistor and thedrain region of the read/write enable transistor extending substantiallythrough a thickness of the active area.
 8. The method of claim 2,wherein removing comprises etching the portion of the bulk semiconductorsubstrate.
 9. A process of forming a capacitor-less memory cell,comprising: forming a read/write enable transistor in a semiconductorsubstrate, the read/write enable transistor comprising a drain regionand a source region; forming a pass transistor in the semiconductorsubstrate, the pass transistor comprising a drain region and a sourceregion, the source region of the pass transistor common in an activearea with the drain region of the read/write enable transistor; andforming the active area to be at least substantially physically isolatedfrom the semiconductor substrate by at least one material and a gate ofthe read/write enable transistor extending at least laterally between aportion of the active area and the source region of the read/writeenable transistor.
 10. The process of forming a memory cell of claim 9,wherein forming a read/write enable transistor comprises forming animplant region in the semiconductor substrate offset from the activeregion in one direction.
 11. The process of forming a memory cell ofclaim 10, wherein forming an implant region in the semiconductorsubstrate offset from the active region in one direction comprisesforming an implant region in the semiconductor substrate offset from theactive region in a direction toward the read/write enable transistor.12. The process of forming a memory cell of claim 9, wherein forming theactive area to be at least substantially physically isolated from thesemiconductor substrate comprises undercutting at least one of a nitridefilm, an oxide film, and the semiconductor substrate.
 13. The process offorming a memory cell of claim 9, wherein forming a read/write enabletransistor comprises forming a gate oxide along at least one side of theactive area.
 14. The process of forming a memory cell of claim 9,wherein forming a pass transistor comprises forming a gate of the passtransistor and coupling the gate of the pass transistor to a word line.